The MQ gas sensors are electro-chemical with a heater.http://en.wikipedia.org/wiki/Electrochemical_gas_sensorThey are sensitive for a range of gasses, and if one of those gasses is in the detector, the conductivity between 'A' and 'B' gets better.With a load resistor, the voltage over the load resistor changes, because of the better conductivity between 'A' and 'B'.That's all.

You can try to calibrate them for a certain amount of gass, some do even calculations with the value to get a percentage. But that is just a wild guess, it is not accurate at all.

Connect sensor with a serial R (depending on sensor used: +/- 3k..30k,potmeter) - to get the analog readout near 500.Leave sensor powed for 2 full days. (readout will drift, so R must be changed)--------Now (sensor powered for looooong time, in clean air) readout is near 500..Study datasheet. to find how R_sensor changes for gas presenceMake your transfer function to convert change in readout to ppm.